The present invention relates to a device for measuring a dimension of an object by means of an opaque, rotating disc provided with at least one translucent spiral slit, and an opaque diaphragm provided with a translucent, substantially linear slit which is arranged adjacent the disc in such a way that a translucent overlap zone is formed between the spiral and linear slits. This overlap zone moves along the linear slit as the disc rotates. Optical means are provided for generating an image of the object being measured on the diaphragm or the disc with the dimension to be measured falling in the zone of the linear slit and parallel to this slit. Also included is a detector for measuring the intensity of light passing through the overlap zone and a circuit for evaluating the measuring signal provided by the detector.
In one known measuring device of this type, the translucent linear slit of the diaphragm is arranged radially to intersect in each case one of the translucent spiral slits having a curvature which is not described in detail (U.S. Pat. No. 3,254,226, FIG. 3). The spiral slits intersect the linear slit at angles that are less than 90.degree. during the rotation of the disc. The overlap zone, therefore, is not rectangular in form, and this results in a measuring signal which rises in a relatively slow manner as the edge of the object is scanned, a condition which may reduce the accuracy of the measurement.
Another known device utilizes a slit in the diaphragm which is made nonlinear in order to make the measurement linear such that the position of the overlap zone along the slit is strictly proportional to the angle of rotation of the disc (German Pat. No. 16 23 323). Such a diaphragm, however, is in practice difficult to manufacture. Furthermore, the slits of this device also intersect at angles other than 90.degree., with the aforementioned disadvantages.
Another known device utilizes a diaphragm having a linear slit which is arranged radially adjacent the disc. A sequence of rectangular openings positioned along a spiral path is provided instead of a spiral slit in the disc (U.S. Pat. No. 3,254,226, FIG. 1). The overlap zone of such an opening with the linear slit is, to be sure, rectangular, but the accuracy of this arrangement is considerably limited by reason of the opaque regions between successive openings in the rotating disc.